A plurality of press-formed materials of steel have been assembled by welding, to be used for automobile frames and the like. In recent years, multistage-formed articles of Al alloy pipes have been used, for the purpose of making the frames or the like into lightweight or modules.
The methods for manufacturing Al alloy pipes are roughly classified into: casting (such as casting and die-casting); and working to make wrought alloys (such as hollow extrusion). An Al alloy pipe manufactured by casting is relatively poor in reliability, since it contains coarse voids or its toughness is low.
An Al alloy pipe manufactured by working to make a wrought Al alloy is used in, for example, front/side frame members of automobiles and frames of motorcycles. Proposed examples of the method for manufacturing an Al alloy pipe using a wrought Al alloy include: (1) applying bending and hydraulic bulge forming to an Al alloy pipe having a circular cross section; (2) applying inner pressure, after bending an Al alloy pipe having a polygonal cross section; and (3) applying pressing and hydraulic bulge forming, by placing an Al alloy pipe in a hydraulic bulge die.
While an Al alloy pipe manufactured by working to make a wrought Al alloy is usually manufactured by mandrel extrusion, as a combination of a die and a mandrel, it can also be manufactured, for example, by port-hole extrusion, by which divided pieces extruded from a port-hole die (a kind of a division die) are fusion welded to form a pipe at the outlet side of the die, or by seam welding, by which the edges of a rolled up sheet are fitted together and welded.
However, there has been such a problem that cracks or the like are liable to be occurred at the bent portions, when a conventional Al alloy pipe as mentioned above is subjected to the second forming step and forming steps thereafter, such as pressing and hydraulic bulge forming, by which the cross sectional shape in the pipe's circumference direction (hereinafter simply abbreviated to “cross sectional shape”) is changed, after the first forming step of bending or the like.
Examples of the Al alloys that have been used in the above-mentioned Al alloy pipes include 1000 series Al alloys, such as 1050 and 1100 alloys; 3000 series Al alloys, such as 3003 and 3004 alloys; 5000 series Al alloys, such as 5052, 5454, and 5083 alloys; 6000 series Al alloys, such as 6063, 6N01, and 6061 alloys, and 7000 series Al alloys, such as 7003 and 7N01 alloys. However, these Al alloys each involve such problems as mentioned below: Insufficient mechanical strength and limited uses, as encountered in Al alloy pipes of the 1000 or 3000 series Al alloys; poor multistage formability, as encountered in Al alloy pipes of the 5000 series Al alloys; poor bending property and multistage formability, as encountered in Al alloy pipes made of the hard 6000 series or 7000 series Al alloys; and poor productivity, as encountered in Al alloy pipes made of the soft 6000 series or 7000 series Al alloys, which require aging after multistage forming, due to their low mechanical strength.